Thermal transfer printing involves the controlled transfer of an ink (e.g., a colorant dispersed in a wax-based material) from a carrier such as a polymer ribbon onto a print medium surface. A thermal transfer printer having a print head with a large number of independently activatable heating elements per unit of length is one prior art apparatus employed for this purpose. The ribbon is placed within the printer such that the carrier side is adjacent to the heating elements, and the ink side is adjacent to a print media support upon which a print medium rests during printing.
To print an image, the print head contacts the ribbon, and ink is transferred to particular locations on the print medium surface when predetermined combinations of heating elements are activated adjacent to those image-forming locations. The ribbon is locally heated by the heating elements to a temperature at or above the melting point of the ink. In this manner, an amount of ink softens and adheres to the print medium at the predetermined locations to form the image.
Color images are printed with a ribbon that typically includes separate panels of differently colored inks such as the subtractive primary colors, yellow, magenta, and cyan. Color printing is accomplished by sequential passes of the print medium past the print head, each pass selectively transferring different colored inks at predetermined times.
Thermal printing ribbons are available with a single yellow, magenta, cyan, or black ink panel (a monochrome ribbon); repeating sets of yellow, magenta, and cyan ink panels; or repeating sets of yellow, magenta, cyan, and black panels (multicolor ribbons). Ribbons are typically supplied on spools that have encoded end caps to communicate to the printer whether the ribbon includes one, three, or four panels. Multicolor ribbons typically have an encoding stripe running along one edge of the length of the ribbon to communicate panel location and panel color data to the printer. Obviously, monochrome ribbons are easier to manufacture, do not require the encoding stripe, and are therefore less costly.
Therefore it would be preferable to use multiple monochrome ribbons rather than a multicolor ribbon for thermal transfer color printing. Other workers have tried using multiple monochrome ribbons, each having an associated thermal print head, for transferring ink from each ribbon to a print medium proper registration of images transferred from each of the different ribbons requires precise alignment of the print heads and accurate positional indexing of the print medium relative to the print heads. Unfortunately, the resulting mechanism is bulky and relatively complex, the electronics drivers are replicated four times, and a higher capacity power supply is required resulting in a cost that offsets any savings associated with using the monochrome ribbons.
To improve multi-pass color printing registration, some printers clamp an edge of the print medium to a media drum and wrap the print medium around the drum for printing. Such an arrangement provides accurate control of print medium positioning relative to the print head(s).
Copending U.S. Pat. Ser. No. 5,305,020 of Gibbons et al. Apr. 19, 1994 for A THERMAL TRANSFER PRINTER HAVING MEDIA PRE-COAT SELECTION APPARATUS AND METHODS, assigned to the assignee of this application, describes such a drum and medium clamp arrangement for use with a multicolor thermal transfer ribbon. Referring to FIG. 1, a thermal transfer printer 10 (hereafter "printer 10") is shown that includes a drum 12 that receives a print medium 14A from a media tray 16. (Print medium 14 is shown in printer 10 at three locations designated by a letter suffix, i.e., 14A, 14B, or 14C.) A leading edge 18 of print medium 14A is fed to a medium clamp 20 that secures print medium 14B to drum 12, which then rotates in a direction indicated by arrow 22 to wrap print medium 14B around drum 12.
Printer 10 also includes a multicolor thermal transfer ribbon 24 suspended between a supply spool 26 and a take-up spool 28. Take-up spool 28 is driven in a direction indicated by arrow 30 with a torque sufficient to feed ribbon 24 through a nip formed between drum 12 and a thermal print head 32 at a rate determined by the rotation of drum 12.
The type of ribbon 24 (black, three, or four panels) is encoded by hub length into a left hub 42 and a right hub 43 on supply spool 26. Hubs 42 and 43 are each of a normal or extended length and selectively activate a left microswitch 44 and/or a right microswitch 45 as listed in Table 1. The states of microswitches 44 and 45 are sensed by a printer controller 46.
TABLE 1 ______________________________________ RIBBON TYPE LEFT SWITCH RIGHT SWITCH ______________________________________ Black On On Three panel Off Off Four panel On Off ______________________________________
Ribbon 24 further includes an opaque encoding stripe 48 having a coded marker 50 at location indicating the boundaries between panels 34, 36, 38, and 40. Coded marker 50 typically is a series of transparent stripes detectable by a photosensor array 52 mounted adjacent to thermal print head 32. The number of stripes in each coded marker 50 indicates to printer controller 46 which 15 of panels 34, 36, 38, or 40 is aligned with thermal print head 32.
In operation, printer 10 receives a print job at a data communications interface 54. The print job is transferred to a system bus 55 that is in communication with printer controller 46, a processor 56, and a memory 58. Processor 56 processes data and commands contained in the print job and transmits control and printing data to printer controller 46. Processor 56 executes the printer driver stored in memory 58 and exchanges data with a PostScript.RTM. interpreter.
After the print job is interpreted by processor 56 and stored as yellow, magenta, and cyan image data in memory 58, printer controller 46 causes print medium 14A to feed from media tray 16 to medium clamp 20 on drum 12. Medium clamp 20 is activated, and drum 12 is caused to rotate such that leading edge 18 of print medium 14B is just past the nip between drum 12 and thermal print head 32. Ribbon 24 is moved by take-up spool 28 until a coded marker 50 is detected by photosensor array 52 indicating that a yellow panel 36 is positioned under thermal print head 32. Drum 12 is rotated one revolution, and yellow panel 36 is moved through the nip while all the image data stored in memory 58 simultaneously drives thermal print head 32, thereby thermally transferring a yellow image to print medium 14B and advancing ribbon 24 such that magenta panel 38 is in the nip. Drum 12 is rotated a second revolution, and magenta panel 38 is moved through the nip while the magenta image data stored in memory 58 simultaneously drives thermal print head 32, thereby thermally transferring a magenta image to print medium 14B and advancing ribbon 24 such that cyan panel 40 is in the nip. The sequence is repeated for the cyan image data until a full color image is transferred and registered on print medium 14B. Drum 12 reverses and rotates in the direction of an arrow 60, releasing medium clamp 20, and feeding print medium 14C from printer 10 by means of a conventional exit path mechanism (not shown).
Printer 10 properly registers high-resolution color images with a relatively simple and inexpensive mechanism. However, if even one dot of ink is transferred from a single color panel to form an image, ribbon 24 must be advanced to a new starting position before printing a subsequent image. This wastes almost three ribbon panels (four if ribbon 24 has a black panel) and requires considerable time to advance ribbon 24 to the new starting position.
U.S. Pat. Ser. No. 4,778,290 issued Oct. 18, 1988 for a PRINTER FOR PRINTING OF A FULL LINE IN SEVERAL COLORS BY INTERCHANGEABLE RIBBON CARTRIDGES describes a thermal printer having a single print head and a drum and medium clamp arrangement that provides multi-pass color registration of ink images transferred from multiple monochrome ribbons to a print medium wrapped around the drum. A carousel of multiple monochrome ribbon cartridges indexes an appropriate ribbon to a printing position adjacent to the drum. The print head, mounted within the carousel, is moved to form a nip between the print head and the drum through which the print medium and the ribbon are frictionally drawn by rotation of the drum. When printing is not desired, the print head is withdrawn a few millimeters to remove the ribbon-driving friction, thereby conserving ribbon material.
The ribbon carousel allows use of monochrome ribbons, improves ribbon utilization, and reduces the time required before a new image can be printed.
However, there are problems associated with mounting the thermal print head inside the carousel. Commercially available thermal print heads are physically large and dissipate a considerable amount of heat. Therefore, the carousel must provide print head clearance between each pair of ribbon spools, space for a print head heat sink, some heat removal means from inside the carousel, and a mechanism for moving the print head into contact with the ribbon. Such requirements result in an unduly large and massive ribbon carousel that requires expensive drum-to-ribbon-to-print head alignment structures and a complex print head moving mechanism.
Therefore, what is needed is a method and apparatus for thermal transfer printing with multiple monochrome ribbons and a commercially available print head but without the mechanical complexity previously associated with print head heat removal, print head movement, and drum-to-ribbon-to-print head alignment.